A Saudi, Moqed was studying law at a university in Saudi Arabia before joining Al-Qaeda in 1999 and being chosen to participate in the 9/11 attacks. He arrived in the United States in May 2001 and helped with the planning of how the attacks would be carried out.

On September 11, 2001, Moqed boarded American Airlines Flight 77 and assisted in the hijacking of the plane so that it could be crashed into the Pentagon.

Moqed was one of the five hijackers who asked for a state identity card on August 2, 2001. On August 24, both Mihdhar and Moqed tried to purchase flight tickets from the American Airlines online ticket-merchant, but had technical difficulties resolving their address and gave up.[4]

Employees at Advance Travel Service in Totowa, New Jersey later claimed that Moqed and Hanjour had both purchased tickets there. They claimed that Hani Hanjour spoke very little English, and Moqed did most of the speaking. Hanjour requested a seat in the front row of the airplane. Their credit card failed to authorize, and after being told the agency did not accept personal cheques, the pair left to withdraw cash. They returned shortly afterwards and paid $1842.25 total in cash.[5] During this time, Moqed was staying in Room 343 of the Valencia Motel. On September 2, Moqed paid cash for a $30 weekly membership at Gold's Gym in Greenbelt, Maryland.

Three days later he was seen on an ATM camera with Hani Hanjour. After the attacks, employees at an adult video store, Adult Lingerie Center, in Beltsville claimed that Moqed had been in the store three times, although there were no transactions slips that confirmed this.[6]

According to the 9/11 Commission Report, Moqed set off the metal detector at the airport and was screened with a hand-wand. He passed the cursory inspection, and was able board his flight at 7:50. He was seated in 12A, adjacent to Mihdhar who was in 12B. Moqed helped to hijack the plane and assisted Hani Hanjour in crashing the plane into the Pentagon at 9:37 A.M., killing 189 people (64 on the plane and 125 on the ground).

The flight was scheduled to depart at 08:10, but ended up departing 10 minutes late from Gate D26 at Dulles.[7] The last normal radio communications from the aircraft to air traffic control occurred at 08:50:51.[8] At 08:54, Flight 77 began to deviate from its normal, assigned flight path and turned south, and then hijackers set the flight's autopilot heading for Washington, D.C.[9] Passenger Barbara Olson called her husband, United States Solicitor GeneralTheodore Olson, and reported that the plane had been hijacked and that the assailants had box cutters and knives.[10] At 09:37, American Airlines Flight 77 crashed into the west facade of the Pentagon, killing all 64 aboard (including the hijackers), along with 125 on the ground in the Pentagon.[11] In the recovery process at the Pentagon, remains of all five Flight 77 hijackers were identified through a process of elimination, as not matching any DNA samples for the victims, and put into custody of the FBI.[12][13]

After the attacks his family told Arab News that Moqed had been a fan of sports, and enjoyed travelling. Additionally, the U.S. announced it had found a "Kingdom of Saudi Arabia Student Identity Card" bearing Moqed's name in the rubble surrounding the Pentagon. They also stated that it appeared to have been a forgery.[14]